Work-related asthma is asthma that is attributable to, or is made worse by, environmental exposures in the workplace. Published estimates of the proportion of adult asthma attributable to occupational factors have varied widely, depending on population, methodology, and definitions, from 2 percent to 33 percent. Occupational asthma is of great public health importance because it is potentially preventable, can cause substantial disability, and in some cases is completely curable. Among adults in the United States, asthma has become a major public health problem, with rates most elevated among low income, urban, African American and Latino sectors of the population, and with substantial evidence suggesting potential occupational contributions to the excess rates. These important sectors of the U.S. population have, however, been inadequately represented in the occupational asthma research literature. A case control study of physician-diagnosed asthma, occupation, industry, and workplace environmental exposures is proposed to evaluate the hypothesis that a substantial component of the asthma burden in a low income, urban, largely minority population is due to occupational factors. The study design addresses a variety of methodologic challenges including healthy worker effects, difficulty contacting and recruiting this potentially high risk population, large numbers of potential etiologic agents, mixed exposures, small workplaces, and low absolute incidence of occupational asthma. The study population is the catchment population of Bellevue Hospital, a general hospital in lower Manhattan, New York City, with busy ambulatory care services that serve low income working communities. Cases and controls will be recruited from among outpatients and inpatients at Bellevue Hospital and interviewed face-to-face or by telephone. Occupation, industry, and occupational exposures will be determined by questionnaire supplemented by a Job Exposure Matrix. Odds ratios (ORs) of association between asthma and specific industrial, occupational, and exposure categories, controlled for major confounders, will be estimated. The ORs will be used to calculate occupation- and industry-specific Attributable Fractions, and an overall Population Attributable Fraction of asthma attributable to occupational factors. New onset occupational asthma and work-aggravated asthma will be investigated separately.